1. Technical Field
The present invention relates to electronic circuits. More particularly, the present invention relates to electronic amplifiers.
2. Description of the Prior Art
Integrated circuits are typically fabricated in one of several different semiconductor technologies, such as bipolar, MOS, GaAs, etc. Each of these technologies offers well recognized design advantages. For example, bipolar devices are fast and capable of handling higher power levels, while MOS devices operate efficiently at low power levels.
Integrated circuit designers are increasingly exploiting the different advantages of each of these technologies by fabricating integrated circuits that include devices of more than one technology type on a common substrate. One such combination that is becoming increasingly important is that of bipolar and CMOS technologies, referred to as `BiCMOS`. For example, it is now common practice to produce BiCMOS amplifiers.
A BiCMOS amplifier typically includes an output stage, consisting of a pair of bipolar transistors arranged in a totem pole or push-pull configuration. That is, the emitters and collectors of the transistor pair are connected in series between a supply voltage and ground terminal, with an amplifier output connected to commonly connected emitter-collector terminals of the transistor pair. The bases of the transistor pair are connected to respond to an input signal, such that the transistor pair provides an output signal having a voltage that swings between a voltage slightly less than the supply voltage and slightly more than ground. This arrangement is best understood with reference to transistors 15 and 17, as shown in FIG. 1.
It is well recognized that a full swing between a supply voltage and ground at the amplifier output is not presently possible with an amplifier of the above design. For example, if V.sub.dd and V.sub.ss are the upper and lower rail voltages respectively, then the maximum available output voltage swing is between V.sub.dd- V.sub.be and V.sub.ss+ V.sub.be, where V.sub.be.apprxeq. 0.6 volts, i.e. V.sub.be= the base-emitter voltage of a bipolar transistor at the onset of conduction.
A less than full output voltage swing is not a critical problem when circuit devices are operated at higher voltages, such as 5 volts, where the devices are less sensitive to the small offsets in voltage swing that the devices themselves introduce. However, the clear trend is to produce smaller and faster devices that use less power and produce less heat. One way this is accomplished is by designing devices that operate at lower voltages, such as 4 volts, 3.3 volts, and even 2 volts. At such device operating voltages, the loss of any portion of the operating voltage swing due to the devices themselves is unacceptable.
A full swing between the supply voltage and ground can be achieved by adding MOS devices to the above amplifier circuit. The output waveform of this type of amplifier is characterized by a fast rise time from V.sub.ss to V.sub.dd- V.sub.be, followed by a slow rise to V.sub.dd. Similarly, a fast transition from V.sub.dd to V.sub.ss+ V.sub.be is followed by a slow tail to V.sub.ss. The steep part of this output waveform corresponds to the drive of the capacitive load by the bipolar transistors, while the slow tails result from the drive by MOS devices, which are much weaker than the bipolar transistors. Thus, although a full swing output signal may be achieved, response time is seriously degraded.
A more recent type of amplifier achieves a fast transition from V.sub.dd to V.sub.ss by extending the conduction range of the pull down bipolar transistor. However, the up-going transition is as described above, i.e. with a slow tail to V.sub.dd. See, for example, K. Yano, M. Hiraki, S. Shukuri, Y. Sawahata, M. Hirao, N. Ohki, T. Nishida, K. Seki, K; Shimohigashi, Quasi-Complementary BiCMOS for Sub-3-V Digital Circuits, 1991 Symposium on VLSI Circuits, The Japan Society of Applied Physics/The IEEE Solid-State Circuits Council (1991).
Until an amplifier having a fast response time and a full output swing from supply voltage (V.sub.dd) to ground (V.sub.ss), and from ground to supply voltage is available, the ability to produce useful BiCMOS integrated circuits having lower operating voltages will be limited.